Semiconductor wafer manufacture today demands ever cleaner environments. Any contamination within the clean room, especially within the process chamber becomes a threat to the profitability of the company in the highly competitive semiconductor manufacturing business. It is a common objective within the industry to strive for a Zero Defect density, D.sub.0. The etching process is one very critical area during which significant contaminants are created in semiconductor manufacturing.
In a plasma etching process, the wafer within a chamber is subjected to a chemical plasma and radio frequency energy is used to catalyze the process. A glow discharge is used to produce chemically reactive substances (atoms, ions, etc.) from a relatively inert molecular gas. These substances then react chemically to remove (etch) the unprotected areas of the target wafer. The very nature of the etching process removes material from the wafer and suspends the by-products in the atmosphere within the processing chamber. These by-products will deposit on virtually any surface available. Although evacuation of the chamber removes the vast majority of etching waste products, some material will inevitably deposit on the surfaces of the chamber itself. Over time, significant deposits accumulate to present a contamination hazard when the deposits flake off and land on the silicon wafer. Even the smallest amount of contamination, in the expensive semiconductor manufacturing process, results in lost time and product rejection. The objective is to have absolutely no contamination on the semiconductor device to interfere with the operation and reliability of the integrated circuits. To combat this, etch chamber manufacturers have prescribed preventive maintenance (cleaning) intervals and procedures. A major objective of the cleaning process is to assure that the environment in the chamber remains as constant as is economically possible, so that each wafer is processed exactly the same as the previous wafer. As contaminants accumulate within the chamber, the consistency of the semiconductor devices from batch to batch is jeopardized.
One manufacturer, Applied Materials, Inc., prescribes a "wet clean" every 3000 wafers processed within their Precision 5000 Mark II chamber. A wet clean requires two certified persons to disassemble the chamber and to literally scrub the accumulated contaminants from the surfaces of the chamber parts with a liquid solvent. This physical cleaning and reassembly is estimated to consume five hours (i.e., 10 manhours) for each chamber, plus one hour for evacuation/pump down, plus two hours for process/particle qualification (seasoning) before production can be resumed. Thus the chamber is out of the production line for preventive maintenance for a minimum of eight clock hours for every 3000 wafers processed. In actual practice, cleaning, seasoning and requalification can easily take as much as 24 clock hours.
The manufacturer's standard of preventive maintenance of a wet clean after every 3000 wafers for the Precision 5000 Mark II chamber is based upon a typical process of etching 0.4 .mu.m polysilicon with 30% overetch (approximately 2 plasma minutes/wafer). Because actual preventive maintenance frequency will differ based upon the particular manufacturing application, radio frequency (RF) hours have been used to track the extent of usage of the chamber. An RF meter measures the cumulative processing, and wet cleans have typically been performed at 40 to 60 RF hours, but also as high as 80 RF hours. In some instances, a special sampling wafer with a known number of particles on it, is placed in the chamber, tested, removed and then the number of additional particles are counted. A standard for particle accumulation is applied, and the decision to wet clean or not to wet clean is made. Regardless, this testing consumes valuable production time. Thus the prior art has relied primarily upon the experience of the operators to dictate the time for wet cleaning before manufacturing process degradation occurs. However, this empirical method is not extremely reliable.
Prior art in situ cleaning processes have been developed in an effort to extend the mean time between cleanings (MTBC). In these processes, the plasma consists of specialized gases under the influence of RF energy that subjects the interior of the chamber to a cleaning action by reacting with the contaminants and removing them by evacuation. One in situ method employs a combination of a fluorinator, typically sulfur hexafluoride SF.sub.6 and oxygen O.sub.2. However, a side effect of this method of in situ cleaning is that the oxygen in the cleaning plasma attacks the specialty plastics, specifically the Vespel parts in the case of the Applied Materials Precision 5000 Mark II. Vespel, as with most plastics, is a hydrocarbon-based plastic and in the presence of oxygen, the Vespel deteriorates (oxidizes). Running this dry clean process is actually destroying some of the chamber parts and thus adds to the contamination problem. As the Vespel parts of the chamber deteriorate , this also increases the maintenance costs of the chamber when the damaged parts have to be replaced.
Accordingly, what is needed in the art is a method for cleaning a contaminated chamber used in the manufacture of semiconductor devices which removes substantially all of the accumulated etch by-products while minimizing damage to the etch chamber parts.